1. Field of the Invention
Exemplary aspects of the present invention relate to a fixing device and an image forming apparatus, and more particularly, to a fixing device and an image forming apparatus including the fixing device for fixing a toner image on a recording medium.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium (e.g., a sheet) according to image data using electrophotography. Thus, for example, a charger uniformly charges a surface of an image carrier; an optical writer emits a light beam onto the charged surface of the image carrier to form an electrostatic latent image on the image carrier according to the image data; a development device supplies toner particles to the electrostatic latent image formed on the image carrier to make the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image carrier onto a sheet or is indirectly transferred from the image carrier onto a sheet via an intermediate transfer member; a cleaner then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the sheet; finally, a fixing device applies heat and pressure to the sheet bearing the toner image to fix the toner image on the sheet, thus forming the image on the sheet.
In one example of a fixing device included in such image forming apparatuses, a pressing member presses against a fixing member to form a fixing nip portion between the pressing member and the fixing member. When a sheet bearing a toner image passes through the fixing nip portion, the fixing member and the pressing member apply heat and pressure to the sheet bearing the toner image to fix the toner image on the sheet. The fixing member is heated by a heater, such as a resistance heater, a halogen lamp heater, an induction heater, or a magnetic heater.
When a plurality of sheets continuously passes through the fixing nip portion to fix toner images on the plurality of sheets in a single print job, a predetermined control for controlling power supply to the heater is performed to maintain temperature of the fixing member at a predetermined level. However, when a plurality of thin sheets having a sheet weight not greater than about 10 g/m2 passes through the fixing nip portion continuously, image density or gloss may vary among toner images formed on the plurality of thin sheets.
For example, a plurality of sheets continuously passing through the fixing nip portion absorbs heat from the pressing member contacting a back side of a sheet, and thereby decreases temperature of the pressing member gradually. The decreased temperature of the pressing member decreases temperature of a front side of the sheet bearing a toner image, degrading image density or gloss of the toner image. Thus, a last sheet of the plurality of sheets has a low image density or gloss while a first sheet of the plurality of sheets has a high image density or gloss, resulting in variation in image density or gloss among toner images formed on the plurality of sheets within a single print job.